1. Field of the Invention
The present invention relates to television signal receiving tuners, and more particularly relates to a television signal receiving tuner that is capable of receiving terrestrial television broadcasts and that has a trap circuit for attenuating frequency-modulation broadcast-band signals.
2. Description of the Related Art
A conventional television signal receiving tuner (hereinafter referred to as simply a tuner) is shown in FIG. 3. In the tuner shown in FIG. 3, one terminal 31 a of a filter 31 is connected to an antenna for receiving a terrestrial television broadcast or a cable for receiving cable television (CATV) signals (neither are shown). Another end 31b of the filter 31 is connected to an input terminal of a VHF (very high frequency) unit 32 and that of a UHF (ultrahigh frequency) unit 33 which are connected in parallel with each other. An output terminal of the VHF unit 32 and an output terminal of the UHF unit 33 are each connected to an integrated circuit 34.The integrated circuit 34 has a mixer, an oscillator, and a PLL (phase locked loop) incorporated therein. The mixer frequency-converts a high frequency signal in the VHF band or in the UHF band into an intermediate frequency signal IF and outputs the intermediate frequency signal IF.
The VHF unit 32 is constructed by sequentially connecting in series a trap circuit 35, a VHF antenna tuning circuit 36, a VHF high frequency amplifier 37 and a VHF interstage tuning circuit 38. Likewise, the UHF unit 33 is constructed by sequentially connecting in series a UHF antenna tuning circuit 39, a UHF high frequency amplifier 40, and a UHF interstage tuning circuit 41.
The trap circuit 35 attenuates FM broadcast signals which interfere with terrestrial television broadcast signals when terrestrial television broadcast signals are received. A parallel resonant circuit constructed with an inductor 42 and a capacitor 43 is connected, via a direct current blocking capacitor 44, in parallel with a switch diode 45, the cathode of which is connected to an input terminal 35a of the trap circuit 35 and the anode is connected to an output terminal 35b thereof. The resonant frequency of the parallel resonant circuit formed by the inductor 42 and the capacitor 43 is set within the FM broadcast-band. The cathode of the switch diode 45 is grounded via a resistor 46 which is provided between the input terminal 35a ground. A trap circuit control voltage Vf is applied via a feeding resistor 47 from the integrated circuit 34 to a node between the switch diode 45 and the resistor 46.
In the VHF unit 32, the VHF antenna tuning circuit 36 outputs a desired VHF band television signal to the VHF high frequency amplifier 37. The VHF high frequency amplifier 37 amplifies the input television signal, and the VHF interstage tuning circuit 38 outputs the amplified television signal to the integrated circuit 34. Operations of the circuits that comprise the UHF unit 33 are the same as those circuits described in the VHF unit 32. Therefore, descriptions of the UHF circuits are omitted.
The integrated circuit 34 outputs a band selection voltage or a tuning voltage in accordance with channel selection data DA input from a television receiver (not shown). The band selection voltage consists of a voltage Vlo for selecting a low band in the VHF band, a voltage Vhi for selecting a high band in the VHF band, and a voltage Vu for selecting the UHF band. When each band is selected, the integrated circuit 34 outputs a voltage of 5 V. The band selection voltages Vlo and Vhi for the VHF band are applied to the VHF antenna tuning circuit 36, the VHF high frequency amplifier 37, and the VHF interstage tuning circuit 38.The band selection voltage Vu for the UHF band is applied to the UHF high frequency amplifier 40. A tuning voltage Vt is applied to the VHF antenna tuning circuit 36, the VHF interstage tuning circuit 38, the UHF antenna tuning circuit 39, and the UHF interstage tuning circuit 41.
For example, in the United States frequency assignments for VHF and UHF band television channels and the FM broadcast-band are shown in Table 1. It should be noticed that the frequency band assigned to channel six is adjacent to that of the FM broadcasting range.
Furthermore, some CATV channels are assigned to the FM broadcast-band (for example, in the United States, between 88 MHz and 108 MHz). Accordingly, the trap circuit 35 is activated when signals for channel six or for particular CATV channels are received; otherwise the trap circuit 35 is not activated.
When signals having frequencies within the terrestrial television broadcasting channel six are received, the trap circuit control voltage Vf, (e.g., a 5 V voltage), is applied via the feeding resistor 47 from the integrated circuit 34 to the trap circuit 35. Since the switch diode 45 is reverse-biased, the switch diode 45 is switched off. With the switched diode 45 switched off, the trap circuit 35 attenuates a signal having a particular frequency, such as 91.25 MHz within the FM broadcast-band, which is in the proximity of the frequency band of channel six.
When the terrestrial television broadcast VHF low band, apart from the band of channel six, or VHF high band is received, the trap circuit control voltage Vf is not applied to the trap circuit 35 and the band selection voltage Vhi or Vlo is applied to the VHF antenna tuning circuit 36. Since the switch diode 45 is forward-biased, the switch diode 45 is switched on. With the switch diode 45 switched on, the input terminal 35a and the output terminal 35b of the trap circuit 35 are short-circuited, allowing high frequency signals to be received by the VHF antenna tuning circuit 36 without being attenuated by the trap circuit 35.
When CATV signals are received, the trap circuit control voltage Vf is not applied to the trap circuit 35 and the band selection voltage Vhi or Vlo is applied to the VHF antenna tuning circuit 36. Since the switch diode 45 is forward-biased, the switch diode 45 is switched on. With the switch diode 45 switched on, the input terminal 35a and the output terminal 35b of the trap circuit 35 are short-circuited, and the CATV signals within the entire VHF band are received by the VHF antenna tuning circuit 36 without being attenuated by the trap circuit 35.
When the above-described conventional tuner receives VHF television signals within the terrestrial television broadcast-band, the integrity of the VHF television signals can be impaired. The frequencies of some of these received signals correspond to twice the frequencies of the FM broadcast-band (between 88 MHz and 108 MHz). It is the second harmonic of these FM broadcast signals that can also interfere with the television signals received by the high frequency amplifier 37 and other components. The quality of the received image is thus degraded if the second harmonic of these FM broadcast signals is not removed.
According to a first aspect of the invention, a television signal tuner capable of receiving at least terrestrial television broadcast signals is provided. The television signal receiving tuner includes a trap circuit for attenuating frequency-modulation (FM) broadcast-band signals that interfere with the terrestrial television broadcast signals. The trap circuit passes select terrestrial television signals while attenuating (or dampening) FM broadcast-band signals that are in proximity to the selected terrestrial television signals, or have frequencies that when doubled coincide with the selected terrestrial television signals. By suppressing interference from adjacent FM broadcast signals and from second harmonics generated from some other FM broadcast signals, degradation in the quality of received images can be minimized.
In a tuner, the attenuated frequency or resonate frequency of the trap circuit changes when with the desired television channel changes. When the desired channel has a frequency that is double the frequency of a FM broadcast-band, second harmonic interference can effect signal quality. Under this condition, suppression of the interference caused by second harmonics of FM broadcast signals can be ensured, which prevents degradation in the quality of the received television images.
The tuner may further include a varactor diode. In one preferred embodiment, the resonant frequency or attenuated frequency of the trap filter may be varied by applying a tuning voltage to the varactor diode that corresponds to the desired channel of frequencies to be received. By changing the reception channel, the resonant frequency or attenuated frequency of the trap filter is changed and the range of interference can be minimized.
In one preferred embodiment, the trap circuit may include a parallel resonant circuit, a switch diode, and a varactor diode in which the parallel resonant circuit, the switch diode, and the varactor diode are connected in parallel with one another. When the desired television signals are in proximity to the FM broadcast-band or the signals are approximately double the frequencies of a FM broadcast-band signal, the switch diode is reversed biased or switched off, and when other television signals are received, the switch diode is forward biased or switched on.
When CATV signals are received, the CATV signals are received without being attenuated by the trap circuit.